Method of controlling a gas cleaning system by measuring a parameter of an absorbent material

ABSTRACT

A gas cleaning system ( 1 ) for removing gaseous pollutants from a hot process gas ( 2   a ) comprises a vessel ( 4 ) for bringing the hot process gas ( 2   a ) into contact with an absorbent material, and a separating device ( 6 ) for separating at least a portion of the absorbent material from the hot process gas ( 2   a ) to form a separated dust material. The gas cleaning system ( 1 ) further comprises a measuring device ( 48, 20, 44, 76 ) for measuring, directly or indirectly, a dust parameter such as a density, and/or a friction, and/or a hygroscopicity, and/or an electrical property of the separated dust material, to obtain a measurement, and a control system ( 46 ) for controlling at least one operating parameter of the gas cleaning system ( 1 ) based on the measurement of the measured dust parameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a divisional application ofU.S. application Ser. No. 14/459,697 filed on Aug. 14, 2014, whichclaims priority to and is a Continuation of International ApplicationNo. PCT/IB2013/051786 filed on Mar. 6, 2013, which claims priority to EPApplication No. 12159041.8 filed on Mar. 12, 2012, each of which isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to a method of controlling a gas cleaningsystem for removing gaseous pollutants from a hot process gas bybringing the hot process gas into contact with an absorbent material andsubsequently separating at least a portion of the absorbent materialfrom the hot process gas thereby forming a separated dust material.

The present invention also relates to a gas cleaning system for removinggaseous pollutants from a hot process gas.

BACKGROUND OF THE INVENTION

In the combustion of a fuel, such as coal, oil, peat, waste, etc., in acombustion plant, such as a power plant or a waste incineration plant, ahot process gas is generated containing among other components, gaseouspollutants, such as hydrogen chloride (HCl) and sulphur oxides, such assulphur dioxide (SO₂). It is normally necessary to remove at least aportion of the gaseous pollutants from the process gas before theprocess gas can be released into the atmosphere, or treated further in,for example, a carbon dioxide (CO₂) compression plant for transport to aCO₂-sequestration plant.

When separating gaseous pollutants, such as hydrochloric acid andsulphur dioxide, from hot process gas, a method is frequently used inwhich a lime-containing absorbent material is introduced into theprocess gas to react with the gaseous pollutants. When the absorbentmaterial reacts with the gaseous pollutants, the gaseous pollutants areconverted chemically or physically into dust material, which is thenseparated in a filter. EP 1 815 903 A1 discloses an example of such amethod, in which lime-containing dust is mixed with water in a mixer andis then introduced into a contact reactor to react with gaseouspollutants of a hot process gas. The dust material formed by suchreaction is separated in a filter and recirculated to the mixer to bemixed again with water for subsequent introduction into the contactreactor.

The dust material formed by reaction of absorbent material with gaseouspollutants may contain some substances, such as calcium chloride, proneto making the dust material sticky, which can cause severe operationaldisturbances in a gas cleaning plant.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of controllinga gas cleaning system for efficient operation with limited or nooperational disturbances.

This object is achieved by means of a method of controlling a gascleaning system for removing gaseous pollutants from a hot process gasby bringing the hot process gas into contact with an absorbent materialand subsequently separating at least a portion of the absorbent materialfrom the hot process gas thereby forming a separated dust material. Themethod of control comprises: measuring, directly or indirectly, at leastone dust parameter selected from a group of dust parameters comprising:a density of the separated dust material, a friction of the separateddust material, a hygroscopicity of the separated dust material, and anelectrical property of the separated dust material to obtain ameasurement, and controlling at least one operating parameter of the gascleaning system based on the measurement from the measured dustparameter of the separated dust material.

An advantage of this method is that the gas cleaning system may becontrolled to operate in an efficient manner with respect to the removalof gaseous pollutants, and/or with respect to the consumption of freshabsorbent, in a manner which causes no or limited operationaldisturbances.

According to one embodiment, the method further comprises comparing themeasurement from the measured dust parameter of the separated dustmaterial to a dust parameter set point, and controlling an operatingparameter to effect a change in the dust parameter of the separated dustmaterial when the measurement of the measured dust parameter indicates arisk of the separated dust material causing operational disturbances. Anadvantage of this embodiment is that the gas cleaning system may becontrolled to operate as efficiently as possible while still keeping theoperating risks associated with sticky dust material at a relatively lowlevel.

According to another embodiment, the method further comprises comparingthe measurement of the measured dust parameter of the separated dustmaterial to a dust parameter set point, and controlling an operatingparameter to effect a change in the dust parameter of the separated dustmaterial when the measured dust parameter indicates no risk of theseparated dust material causing operational disturbances. An advantageof this embodiment is that the method involves utilization ofpossibilities of operating the gas cleaning system in a more efficientmanner, by controlling the gas cleaning system to operate at a new dustparameter value closer than the presently measured dust parameter valueto a value of the dust parameter at which operational disturbances areto be expected.

According to one embodiment, the at least one operating parameter isselected from a group comprising: supply of fresh absorbent to the gascleaning system, supply of water to the gas cleaning system, degree ofrecirculating separated dust material to the gas cleaning system, andtemperature of the hot process gas inlet to the gas cleaning system. Anadvantage of these parameters is that they are relatively easy toadjust, and have a relatively large and relatively fast influence on thedust parameter of the separated dust material. Hence, control of one ormore of the operating parameters may achieve a relatively large and fastinfluence on dust material stickiness.

According to one embodiment, the method comprises measuring as a dustparameter of the separated dust material, the density of the separateddust material directly by means of a density meter. An advantage of thisembodiment is that a relatively fast and relatively reliable measurementof the separated dust material density can be obtained.

According to another embodiment, the method comprises measuring, as adust parameter of the separated dust material, the density of theseparated dust material and/or the friction of the separated dustmaterial, indirectly by means of measuring an operating parameter of adevice handling the separated dust material. An advantage of thisembodiment is that a relatively low cost and relatively low maintenancemeasurement of density and/or friction of the separated dust materialcan be obtained by analysing operation of a device handling theseparated dust material, since no or only limited additional equipmentis required. According to a preferred embodiment, the method embodimentfurther comprises measuring the density and/or the friction of theseparated dust material indirectly by means of measuring power drawn bya motor of a device handling the separated dust material. An advantageof this embodiment is that it is relatively easy to accurately measurethe motor power draw, and such power draw level has been found to be areliable indicator of the density and/or friction of the dust materialhandled using the motor.

According to one embodiment, the method further comprises measuring asthe dust parameter an electrical property of the separated dust materialselected from a group of electrical properties consisting ofconductivity, resistivity, and capacitance. An advantage of thisembodiment is that each of the mentioned electrical properties can bemeasured at a relatively low cost, and has a reliable correlation to theseverity of operational disturbances of the gas cleaning system.

According to one embodiment, the method further comprises selecting aset point for the dust parameter of the separated dust material byoperating the gas cleaning system at various values of the dustparameter of the separated dust material and evaluating thecorresponding operational disturbances. An advantage of this embodimentis that a suitable set point can be determined relatively accurately fora specific gas cleaning system, taking into account the specificconditions under which the specific system operates.

According to one embodiment, the method further comprises measuring thedust parameter of the separated dust material within 30 minutes ofseparating the dust material from the hot process gas. An advantage ofthis embodiment is that variations in the operation of the gas cleaningsystem are quickly registered, such that actions to adjust the dustparameter can be taken before any operating disturbances relating tosticky dust material occurs. Furthermore, measuring the dust parameterquickly after the dust material is separated reduces the risk that thedust parameter measurement may be altered by the storing conditions ofthe separated dust material.

A further object of the present invention is to provide a gas cleaningsystem that provides for relatively efficient operation with limited orno operational disturbances.

This object is achieved by means of a gas cleaning system for removinggaseous pollutants from a hot process gas comprising a vessel forbringing the hot process gas into contact with an absorbent material,and a separating device for separating at least a portion of theabsorbent material from the hot process gas to form a separated dustmaterial. The gas cleaning system comprises a measuring device formeasuring, directly or indirectly, at least one dust parameter selectedfrom a group of dust parameters consisting of a density of the separateddust material, a friction of the separated dust material, ahygroscopicity of the separated dust material, and an electricalproperty of the separated dust material, to obtain a measurement, and acontrol system for controlling at least one operating parameter of thegas cleaning system based on the measurement of the measured dustparameter of the separated dust material.

An advantage of this gas cleaning system is that efficient removal ofgaseous pollutants may be obtained with no or limited operationaldisturbances.

According to one embodiment, the gas cleaning system further comprises amotor driving a device handling the separated dust material, the controlsystem adapted for sensing power drawn by the motor to indirectlymeasure, using the power draw, a dust parameter of the separated dustmaterial, such as a density of the separated dust material, and/orfriction of the separated dust material. An advantage of this embodimentis that handling, e.g. mixing or transporting, of the separated dustmaterial and measuring the density and/or the friction, indirectly, ofthe separated dust material can be obtained using one and the samedevice, namely the motor.

According to one embodiment, the system comprises a scale for measuringa weight of a sample of a defined volume of the separated dust materialto determine as a dust parameter of the separated dust material, thedensity of the separated dust material. An advantage of this embodimentis that a relatively efficient and yet reliable density measurement canbe obtained.

Further objects and features of the present invention will be apparentfrom the following detailed description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail below with reference to theappended drawings in which:

FIG. 1 is a schematic side view and shows a gas cleaning systemaccording to a first embodiment.

FIG. 2 is a schematic process chart of an embodiment of a method inaccordance with the present disclosure.

FIG. 3a is a diagram illustrating an example of a correlation betweenmeasured chlorides and density.

FIG. 3b is a diagram illustrating an example of a correlation betweendensity and estimated operational disturbances.

FIG. 4a is a diagram illustrating an example of a correlation betweenmeasured chlorides and conductivity.

FIG. 4b is a diagram illustrating an example of a correlation betweenconductivity and estimated operational disturbances.

FIG. 5 is a schematic side view of a gas cleaning system according to asecond embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a gas cleaning system 1. The gas cleaning system 1 isadapted for cleaning a hot process gas 2 a in the form of a flue gas 2formed during, for example, incineration of a waste, such as municipalor industrial waste, in a waste incinerator, not shown, or duringcombustion of a fuel, such as coal, oil or peat. The flue gas 2 maycontain dust, such as fly ash, and gaseous pollutants, such as sulphurdioxide (SO₂) and hydrochloric acid (HCl) produced during theincineration or combustion process. The gas cleaning system 1 comprisesa contact reactor 4, a dust separator in the form of a fabric filter 6,sometimes referred to as a bag house, and a mixer 8. An example of afabric filter 6 can be found in U.S. Pat. No. 4,336,035.

The flue gas 2 first passes via a duct 10 to the fluidly connectedcontact reactor 4. In the contact reactor 4, a particulate absorbentmaterial reactive with the gaseous pollutants of the flue gas 2 isintroduced in a moistened state into the flue gas 2. Upon contact withthe flue gas 2, the moistened particulate absorbent material convertsthe gaseous pollutants into a solid material in the form of separabledust. The flue gas 2 and the separable dust flows via fluidly connectedduct 12 to the fluidly connected fabric filter 6 where the separabledust is separated from the flue gas 2 to form cleaned flue gas 14 and aseparated dust material. The cleaned flue gas 14 leaves the fabricfilter 6 via fluidly connected duct 16 and is emitted into the ambientair via a fluidly connected stack, not shown. In accordance with analternative embodiment, the cleaned flue gas 14 may be forwarded to aCO₂-compression plant for compression and subsequent CO₂-sequestration.

The fabric filter 6 is provided with a dust hopper 18 for collecting theseparated dust material. Via dust hopper 18, the separated dust materialreaches a connected common dust storage system 20. A minor portion ofthe separated dust material separated by fabric filter 6 is dischargedas discharge dust from the dust storage system 20 via a fluidlyconnected pipe 22. The pipe 22 ends in a rotary discharger 24. Therotary discharger 24 feeds the discharge dust into a silo (not shown)for storage or feeds the discharge dust directly to a truck fortransportation to e.g. a landfill.

While the minor portion of discharge dust is discharged from the system1 by means of the rotary discharger 24, the major portion of theseparated dust material is fed, via a fluidly connected pipe 26 to themixer 8. The mixer 8 is provided with a fluidising cloth or net 28.Pressurised air is introduced to the mixer 8 vertically below the cloth28 via a fluidly connected pipe 30 in order to fluidise the separateddust material in the mixer 8. Water is added to the mixer 8 via afluidly connected pipe 32 and fresh absorbent, such as burnt lime (CaO)or hydrated lime (Ca(OH)₂), is added to the mixer 8 via a fluidlyconnected pipe 34. The water moistens the separated dust material andslakes any burnt lime (CaO) to form hydrated lime (Ca(OH)₂). Furtheradditives, such as activated carbon, may be fed to mixer 8 via a fluidlyconnected pipe 36. The mixer 8 is provided with an agitator 38 forthoroughly mixing the separated dust material with water, freshabsorbent and any further additives. The agitator 38 may compriseelliptic plates 40 arranged on a revolvable shaft 42. A motor 44 isconnected to the revolvable shaft 42 and is arranged for drivingagitator 38. A thorough description of a mixer 8 useful in the presentinvention may be found in WO 97/37747 A1.

The separated dust material, moistened by the water added via the pipe32, is introduced at the bottom 4 a of the contact reactor 4 and isthereby introduced into the flue gases 2 as an absorbent material forabsorbing further gaseous pollutants from the flue gases 2. It will beappreciated that the separated dust material is circulated many times inthe gas cleaning system 1. The minor portion of discharge dustdischarged via the rotary discharger 24 keeps the amount of dust in thesystem 1 constant. The flue gas 2 contains some fly ash, reactionproducts are formed continuously in the gas cleaning system 1, and freshabsorbent and additives are added such that there is a continuousaddition of dust into the system 1 which is compensated for by thedischarge of discharge dust via the discharger 24. Thus the overallamount of dust in the system 1 is rather constant over time.

The system 1 comprises a control system 46 arranged for controlling theoperation of the gas cleaning system 1. The control system 46 receives,in accordance with a first embodiment, measurement signals from adensity meter 48. The density meter 48 is arranged for measuring a dustparameter, i.e., the density of the separated dust material of the gascleaning system 1. The density meter 48 may, for example, be a Model DSGNuclear Density detector, which is available from Ohmart/Vega,Cincinnati, USA. A vertical measuring pipe 50 is arranged upstream withregard to the flow of discharge dust, of the rotary discharger 24. Onregular intervals, for example once every 2 to 120 minutes, the controlsystem 46 controls the rotary discharger 24 to cause the rotarydischarger 24 to stop discharging discharge dust. The rotary discharger24 so stops and causes the measuring pipe 50 to be filled with dischargedust. When the measuring pipe 50 is filled with discharge dust, thedensity meter 48 measures the density, for example in units of kg/m³, ofthe discharge dust contained in the measuring pipe 50. A signal relayingthe measurement information from the measured discharge dust density issent from the density meter 48 to the control system 46.

The control system 46 compares the measurement measured by the densitymeter 48 to a set point for the density, to determine whether or not theseparated dust material circulating in the gas cleaning system 1 issticky. If the density measurement is above the set point, then actionsmay be initiated by control system 46 to reduce the density of separateddust material, to reduce the risk of operational disturbances associatedwith sticky dust. If the density measurement is below the set point,then actions may be initiated by control system 46 to increase thedensity of the separated dust material, since a high density dust isoften more efficient in removing gaseous pollutants, such as HCl andSO₂, from the flue gas 2.

In accordance with a first embodiment, the control system 46 may controlthe supply amount of fresh absorbent to the mixer 8 via pipe 34. Acontrol valve 52 may be arranged on the pipe 34. If the densitymeasurement of the discharge dust is too high in relation to the setpoint therefor, then the control system 46 may control the valve 52 to“open” or adjust to a position such that more fresh absorbent issupplied to the mixer 8. An increased supply of fresh absorbent to themixer 8 via pipe 34 tends to cause a reduction of the density of theseparated dust material. On the other hand, if the density measurementis too low in relation to the set point therefor, then the controlsystem 46 may control the valve 52 to “close” or adjust to a positionsuch that less fresh absorbent is supplied to the mixer 8. A reducedsupply of fresh absorbent to the mixer 8 via pipe 34 tends to cause anincrease of the density of the separated dust material.

In accordance with a second embodiment, the control system 46 maycontrol the supply amount of water via pipe 32 by means of a controlvalve 54, and/or control the rate of separated dust materialrecirculation in system 1 by controlling a valve 56 arranged on pipe 26and by controlling the rotary discharger 24. The amount of watersupplied to mixer 8 and the rate of separated dust materialrecirculation through system 1 influences the separated dust materialdensity. Greater amounts of water supplied to mixer 8 increases thedensity of the separated dust material, and higher rates ofrecirculation of separated dust material reduces the density of theseparated dust material.

In accordance with a third embodiment, the control system 46 may controlthe temperature of the flue gas 2 at inlet 1 a to the gas cleaningsystem 1. Such may be accomplished by the control system 46 controllinga boiler 58 arranged upstream with regard to the flow of flue gas 2through gas cleaning system 1. A higher temperature of the flue gas 2 atinlet 1 a results in less water being bound to the circulating separateddust material, and a lower density of the separated dust material. Alower temperature of the flue gas 2 at inlet la results in more waterbeing bound to the circulating separated dust material, and a higherdensity of the separated dust material.

The control system 46 may control one parameter such as supply of freshabsorbent, supply of water, rate of separated dust materialrecirculation, and temperature of inlet 1 a flue gas 2, or may controlseveral of those parameters in combination. The parameter(s) suitableand selected for control may also vary. For example, if theconcentration of gaseous pollutants of the inlet 1 a of flue gas 2 ishigh, then correcting a high density of the separated dust material byincreasing the temperature of the inlet 1 a flue gas 2 is lessdesirable, since an increased temperature of the inlet 1 a flue gas 2tends to reduce system 1 efficiency of removing gaseous pollutants. Insuch a situation, the control system 46 may, instead of controlling thetemperature of the inlet 1 a flue gas 2, control the supply amount offresh absorbent via pipe 34 to increase in order to reduce the densityof the circulating separated dust material, without reducing the removalefficiency of system 1 with respect to gaseous pollutants. A first gassensor 60 may be arranged for obtaining a measurement by measuring theconcentration of gaseous pollutants, such as HCl and SO₂, in the inlet 1a flue gas 2, and a second gas sensor 62 may be arranged for obtaining ameasurement by measuring the concentration of gaseous pollutants in thecleaned flue gas 14. The control system 46 may receive measurementsignals from the first and second gas sensors 60, 62, and may utilizethe measurement information received when determining action(s) forcontrolling the density of the separated dust material.

The density meter 48 may be utilized for directly measuring the densityof the separated dust material of the gas cleaning system 1 to obtain ameasurement. In accordance with an alternative embodiment, a weighingcylinder 64 may be arranged downstream with regard to flow of dischargedust through pipe 22, of the rotary discharger 24. The weighing cylinder64 has a specific volume, and is arranged on a scale 66. A level meter68 is arranged for detecting when the weighing cylinder 64 is full ofdischarge dust. When “full cylinder” is detected, a signal indicating aweight measurement as measured by scale 66 is sent to control system 46which determines, based on the volume of cylinder 64 and the weightmeasurement, the density of the separated dust material. A pneumatictransport system 70 empties the weighing cylinder 64 and forwards thedischarge dust to disposal.

In accordance with a yet further embodiment, the control system 46receives a measurement signal from the motor 44 driving the agitator 38.The signal from the motor 44 indicates a measurement of the power drawnby the motor 44 for driving the agitator 38. The power drawn by themotor 44 is an indirect measure of the density of the separated dustmaterial present in the mixer 8. An increasing power draw by motor 44correlates to an increasing density of the separated dust material. Inthis embodiment, the motor 44 will have the function of a measuringdevice, measuring, indirectly, the density of the separated dustmaterial handled in the mixer 8, to obtain a density measurement. Hence,the control system 46 may, as described hereinbefore, control, based onthe density measured indirectly from the power drawn by motor 44, one ormore of the parameters regarding supply of fresh absorbent, supply ofwater, rate of recirculating dust, and temperature of inlet 1 a flue gas2. It will be appreciated that measuring the separated dust materialdensity indirectly by measuring the power draw of motor 44 meansmeasuring the density of separated dust material during the mixing ofthe same with fresh absorbent, water, and any other additives. Using thedensity meter 48 means, on the other hand, measuring only the separateddust material. As such, measuring the separated dust material densityindirectly from the operation of the mixer 8 may provide a feed-backlike signal indicating the result of actions taken, such as increasingthe supply of fresh absorbent, while measuring the separated dustmaterial density using density meter 48 may provide a feed-forward likesignal, indicating the need for actions to correct or adjust thedensity.

In accordance with another indirect method of measuring the density ofthe separated dust material, the control system 46 may receive a signalrelating to the function of the dust storage system 20. The dust storagesystem 20 may be a fluidised system comprising a fluidising cloth or net72. Pressurised air is introduced into dust storage system 20 fromvertically below the cloth 72 via a pipe 74 in order to fluidise thedust in the dust storage system 20. A pressure meter P could be arrangedin the pipe 74 for measuring the air pressure required for fluidisingthe separated dust material contained in the dust storage system 20, toobtain an air pressure measurement. The air pressure measurementobtained using meter P could be utilized as an indirect measure of thedensity of separated dust material of gas cleaning system 1, with arelatively high air pressure measurement correlating to a relativelyhigh separated dust material density. A level meter L could be arrangedin the dust storage system 20 to measure and obtain a measurement of thelevel or amount of fluidized separated dust material therein. The levelor amount measured by meter L could be utilized as an indirect measureof the density of separated dust material of gas cleaning system 1, witha relatively low level or amount correlating to a relatively highseparated dust material density. Hence, the dust storage system 20 couldfunction as a measuring device, measuring, indirectly, the density ofthe separated dust material.

In accordance with a further alternative embodiment, the dust parametermeasured with respect to the separated dust material is an electricalproperty of the separated dust material. The electrical property of theseparated dust material could be selected from a group of electricalproperties consisting of conductivity, resistivity, and capacitance. Inone embodiment a capacitance meter 76 is arranged for measuring thecapacitance of separated dust material in a vertical measuring pipe 78arranged upstream with regard to the flow of discharge dust in pipe 22,of the rotary discharger 24. On regular intervals the control system 46controls the rotary discharger 24 to stop, causing the measuring pipe 78to fill up with discharge dust. When the measuring pipe 78 is filledwith discharge dust, capacitance meter 76 measures the capacitance, forexample in the unit Farad, of the discharge dust contained in themeasuring pipe 78. A signal containing measurement information as to themeasured capacitance is sent from the capacitance meter 76 to thecontrol system 46. One example of a capacitance meter is Agilent U1701Aavailable from Agilent Technologies Inc, Santa Clara, US.

The control system 46 compares the capacitance measurement as measuredby the capacitance meter 76 to a set point for the capacitance todetermine whether or not the separated dust material circulating in thegas cleaning system 1 is sticky. If there is a risk that the separateddust material circulating in the gas cleaning system 1 is sticky, thenmeasures similar to those described hereinbefore with regard to adensity measurement above that of the density set point, may beinitiated to decrease the risk of operational problems. If, on the otherhand, the separated dust material circulating in the gas cleaning system1 is at no risk of stickiness, then measures similar to those describedhereinbefore with regard to a density measurement below that of thedensity set point, may be initiated to increase the efficiency of thegas cleaning system 1.

In accordance with an alternative embodiment, the friction of theseparated dust material could be measured to obtain a measurement andutilized as the dust parameter based on which the at least one operatingparameter of the gas cleaning system 1 is controlled. The friction ofthe separated dust material indicates how “pasty” the dust is. Hence,the friction measurement is indicative of the risk of operationaldisturbances. A relatively high friction measurement for the separateddust material indicates an increased risk of operational disturbancescompared to a relatively low friction measurement for the separated dustmaterial. There are various ways of measuring the friction of theseparated dust material. For example, the friction of the separated dustmaterial could be measured as the internal friction of the separateddust material, and as the wall friction of the separated dust material,both in accordance with, for example, measurement standard ASTM D6773.Furthermore, the friction of the separated dust material could bemeasured as the powder flowability, in accordance with, for example,measurement standard ASTM D6128. Furthermore, the friction of theseparated dust material could also be measured as the viscosity of thefluidized separated dust material in the mixer 8.

Still further, the friction of the separated dust material could bemeasured indirectly, by measuring the power drawn by a device handlingthe separated dust material. A separated dust material with a relativelyhigh friction measurement has a relatively large resistance to handling.Hence, the power drawn by, for example, the motor 44 driving theagitator 38 increases with increasing friction of the separated dustmaterial. Hence, the friction of the separated dust material could bemeasured indirectly by measuring the power drawn by the motor 44, andcorrelating the power measurement to the friction of the separated dustmaterial. Thus, according to this alternative embodiment, the controlsystem 46 receives a signal from the motor 44 driving the agitator 38,and utilizes such signal conveying the friction measurement of theseparated dust material present in the mixer 8, in controlling, based onthe indirect friction measurement, one or more of the above-mentionedoperating parameters of the gas cleaning system 1. Still further, thefriction measurement of the separated dust material could be indirectlymeasured by measuring the power drawn by the rotary discharger 24, or bymeasuring the power drawn another device handling the separated dustmaterial.

In accordance with a further alternative embodiment the hygroscopicityof the separated dust material could be measured and utilized as thedust parameter based on which at least one operating parameter of thegas cleaning system 1 is controlled. The hygroscopicity measurement ofthe separated dust material indicates how prone the separated dustmaterial is to adsorb moisture, and hence, indicates the level of riskof operational disturbances. A relatively high hygroscopicitymeasurement indicates that the separated dust material is very prone toadsorb moisture and indicates a relatively high risk of operationalproblems. The hygroscopicity of the separated dust material could bemeasured by the apparatus HMA available from Waltti Electronics Ltd,Kuopio, Finland. The control system 46 could control one or more of theabove mentioned operating parameters based on the hygroscopicitymeasurement.

It will be appreciated that a combination of direct and indirect methodsof density or friction or electrical property measurements could beutilized to increase the accuracy of the separated dust materialparameter assessments, and to reduce the risk of separated dust materialparameter measurement failure. It is also possible to combinemeasurements of two or more different separated dust materialparameters, such as measurements of density and of an electricalproperty of the separated dust material, for that same purpose.

FIG. 2 is schematic process chart illustrating one embodiment of amethod of controlling the operation of the gas cleaning system 1illustrated in FIG. 1. In the process chart of FIG. 2, reference isgiven to density measurement, but it will be appreciated that theoperation of the gas cleaning system 1 could be based, in a similarmanner, on measurement of an electrical property, of friction or ofhygroscopicity of the separated dust material.

In a step “A”, the density of the separated dust material is measured toobtain a density measurement. Such density measurement could, asdescribed hereinbefore, be direct, using density meter 48, or weighingcylinder 64 and scale 66, or indirect, using signals from motor 44,pressure meter P or level meter L.

In an optional step “B”, the concentration of gaseous pollutants, suchas HCl and SO₂, is measured in the flue gas 2 at inlet 1 a, and/or inthe cleaned gas 14.

In a step “C”, the density measurement of step A is compared to a setpoint for the density. The density set point corresponds to a densitythat is preferably not exceeded to avoid operational disturbancesrelated to sticky dust.

In a step “D”, actions are initiated if the density measurement ishigher than the density set point. Such actions may, as describedhereinbefore, include increasing the supply of fresh absorbent,increasing the temperature of the flue gas 2 at inlet 1 a, etc.Optionally, the actions are initiated with consideration to theconcentration of gaseous pollutants as measured in step B. For example,if it has been concluded in step B that the concentration of gaseouspollutants in the clean gas 14 is close to a limit concentrationtherefore, then increasing the temperature of the flue gas 2 at inlet 1a may not be a suitable action to implement, at least not taken alone,since an increased flue gas 2 temperature at inlet 1 a tends to reducethe removal efficiency of gaseous pollutants. Thus, in such a case, anincreased supply of fresh absorbent may be a more suitable action toreduce separated dust material density.

In a step “E”, actions are initiated if the density measurement is lowerthan the density set point. Such actions may, as described hereinbefore,include reducing the supply of fresh absorbent, reducing the temperatureof flue gas 2 at inlet 1 a, etc. Optionally, the actions are initiatedwith consideration given to the concentration of gaseous pollutants asmeasured in step B. For example, if it has been concluded in step B thatthe concentration of gaseous pollutants in the clean gas 14 is close toa limit concentration therefore, then reducing the temperature of theflue gas 2 at inlet 1 a may be a suitable measure, since a reducedtemperature tends to increase the removal efficiency of gaseouspollutants. A reduced supply of fresh absorbent may also be a suitablemeasure, for cost reasons, and may be combined with the reduction intemperature of flue gas 2 at inlet 1 a to increase the density of theseparated dust material.

Generally, the most efficient manner, with respect to removal efficiencyof gaseous pollutants, and operating costs, including the consumption offresh absorbent, is to operate at high concentrations of chloride (Cl⁻)in the separated dust material, since chlorides make the separated dustmaterial more efficient in removing gaseous pollutants from the flue gas2. It has been found that there is a correlation between theconcentration of chlorides and the density of the separated dustmaterial.

FIG. 3a is a diagram illustrating an example of a correlation betweenmeasured chlorides and density. The Y-axis of the diagram depicts theconcentration of chlorides in unit g/kg, in the separated dust material,and the X-axis depicts the corresponding density in unit kg/m³, of theseparated dust material. As illustrated, the chloride concentrationincreases steeply at densities above about 900 kg/m³. It will beappreciated that the correlation between chlorides and density is plantspecific, influenced by such factors as amount of fly ash and gaseouspollutants in flue gas, rate of separated dust material recirculation,type and amount of fresh absorbent supplied, etc. Hence, to achieverelatively high efficiency in removing gaseous pollutants, it isbeneficial to operate the system 1 at the highest possible density ofseparated dust material, since a relatively high density of separateddust material comprises a relatively high concentration of chlorides,and results in more efficient gaseous pollutant removal.

FIG. 3b is a diagram illustrating an example of a correlation betweenseparated dust material density and estimated operational disturbancesin the gas cleaning system 1. The operational disturbances may, forexample, start with discordant sounds from the rotary discharger 24 andan increased pressure drop with respect to the flue gas over the fabricfilter 6. If the density of the separated dust material is not lowered,the operational disturbances may increase to include dust clogginginside of fabric filter 6, in dust storage system 20, in mixer 8, and induct 4 and pipes 22, 26 connecting those devices. Such dust cloggingwithin system 1 may require shutting down the plant for maintenance,cleaning and repair, which should be avoided when possible. In FIG. 3b ,operational disturbances on the Y-axis have been graded from 1 to 4,where “1” indicates a normal operation of relatively low risk that couldcontinue for months without operational disturbances, and “4” indicatesoperation of relatively high risk requiring an immediate shut down ofthe gas cleaning system 1 due to operational disturbances. Grades “2”and “3” indicate operation risk levels intermediate to grades “1” and“4” described above. The X-axis of FIG. 3b depicts the correspondingdensity of the separated dust material. It will be appreciated that thecorrelation between operational disturbances and separated dust materialdensity is plant specific influenced by such factors as amount and typeof fly ash and gaseous pollutants in flue gas, rate of separated dustmaterial recirculation, type and amount of fresh absorbent supplied,etc. To determine a suitable separated dust material density, empiricaltesting can be done. Such tests could include increasing the density ofthe separated dust material stepwise until the first signs ofoperational disturbances occur, such as for example, discordant soundsfrom the rotary discharger 24. When discordant sounds from thedischarger 24 occur, the separated dust material density is decreasedagain until the discordant sounds subside. The density of the separateddust material at the point of discordant sound elimination may then beused as a set point density used by the control system 46 to determineif any actions need implementation to increase or decrease the separateddust material density to avoid operational disturbances. In the exampledepicted in FIG. 3b , the set point for separated dust material densitycould be selected as 1040 kg/m³. Such separated dust material densitywould be expected to result in the most efficient operation of system 1with respect to consumption of fresh absorbent and removal of gaseouspollutants, without causing undesirable operational disturbances.

FIG. 4a is a diagram similar to that of FIG. 3a , that illustrates anexample of a correlation between measured chlorides and electricalconductivity. The Y-axis of the diagram depicts the concentration ofchlorides in unit g/kg, in the separated dust material, and the X-axisdepicts the corresponding electrical conductivity in unit Siemens/m, ofthe separated dust material. Conductivity data for the diagram of FIG.4a may be obtained by measuring the electrical conductivity for a numberof separated dust material samples of different chloride concentration.

FIG. 4b is a diagram illustrating an example of a correlation betweenelectrical conductivity and estimated operational disturbances occurringin the gas cleaning system 1. Similar to FIG. 3b , the operationaldisturbances are graded from 1 to 4, where “1” indicates a normaloperation of relatively low risk of operational disturbances, and “4”indicates a state of operation of relatively high risk for operationaldisturbances requiring an immediate shut down of the gas cleaning system1. The control system 46 initiates actions to reduce the risk ofoperational disturbances when the measured electrical conductivityexceeds a conductivity set point.

FIG. 5 illustrates a gas cleaning system 101 according to a secondembodiment. The gas cleaning system 101 comprises a spray dryer absorber104, a dust separator in the form of a fabric filter 106, sometimesreferred to as a bag house, and a mixing system 108.

Flue gas 102 flows via a fluidly connected duct 110 to the spray dryerabsorber 104. The spray dryer absorber 104 comprises a spray dryerchamber 105 and a number of dispersers 107, 109 mounted at a roof 111 ofthe spray dryer chamber 105. Each disperser 107, 109 comprises anatomizer 113, which may be, for example, a rotary atomizer, an exampleof which is described in U.S. Pat. No. 4,755,366, or a nozzle, operativefor atomizing an absorbent material in the form of an absorption liquidsupplied from mixing system 108 via a fluidly connected pipe 115.

Each disperser 107, 109 is provided with a flow directing device 117operative for mixing an amount of flue gas 102 flowing through fluidlyconnected duct 110 with absorption liquid atomized by the respectiveatomizer 113.

The mixture of flue gas 102 and absorption liquid travels downwardlywithin interior 105a of the spray dryer chamber 105, causing a reactionof gaseous pollutants with absorbent material of the absorption liquid,and evaporation of the water content of the absorption liquid. A drysolid material in the form of separable dust is, as an effect of suchreaction and evaporation, formed in interior 105a of the spray dryerchamber 105. The flue gas 102, from which most of the gaseous pollutantshave been removed, leaves the spray dryer absorber 104 via a fluidlyconnected duct 112. The flue gas 102 flows through duct 112 to thefabric filter 106. As an alternative, the fabric filter 106 may besubstituted with an electrostatic precipitator or any other suitablefiltering device. The fabric filter 106 removes most of the separabledust from the flue gas and forms a separated dust material and a cleanedflue gas. The cleaned flue gas may then be admitted to the ambient airvia a fluidly connected clean gas duct 116.

Separable dust collected at a bottom 121 of the spray dryer chamber 105is a first discharged dust portion discharged from the gas cleaningsystem 101 via a fluidly connected pipe 122. The pipe 122 is in fluidcommunication with a first rotary discharger 123, which feeds the firstdischarged dust portion, via a fluidly connected discharge pipe system125, into a silo 126 for storing end product, or feeds the dischargeddust directly to a truck for transportation to e.g. a landfill. Theseparated dust material collected in the fabric filter 106 is a seconddischarge dust portion discharged from the gas cleaning system 101 via afluidly connected pipe 127. The pipe 127 is in fluid communication witha second rotary discharger 124, which feeds the second discharged dustportion, via the fluidly connected discharge pipe system 125 into thesilo 126, or feeds the discharged dust directly to a truck fortransportation to e.g. a landfill.

The mixing system 108 comprises a tank 128 for mixing water supplied viaa fluidly connected pipe 132 with fresh absorbent, such as burnt lime,(CaO), or hydrated lime, (Ca(OH)₂), supplied via a fluidly connectedpipe 134. The mixing of water and fresh absorbent in the tank 128 formsthe absorption liquid supplied via fluidly connected pipe 115 to thedispersers 107, 109.

The gas cleaning system 101 comprises a control system 146, whichreceives measurement information in the form of signals from a densitymeter 148 arranged for measuring the density of the separated dustmaterial collected in the fabric filter 106. The density meter 148measures the density of separated dust material in a fluidly connectedvertical measuring pipe 150 arranged upstream with regard to the flow ofdust through pipe 127, of the second rotary discharger 124.

The control system 146 compares the measurement of the separated dustmaterial density as measured by the density meter 148 to a set point forthe separated dust material density, to determine whether or not theseparated dust material of the gas cleaning system 101 is sticky. If theseparated dust material density measurement is above the set point, thenactions may be initiated by control system 146 to reduce the separateddust material density, to reduce the risk of operational disturbancesassociated with sticky dust.

In accordance with a first embodiment, the control system 146 maycontrol the supply level of fresh absorbent to the tank 128 via fluidlyconnected pipe 134 by controlling a control valve 152. If themeasurement for the density of the separated dust material, as measuredby density meter 148, is too high as compared to the set point, then thecontrol system 146 may signal adjustment of the valve 152 to allow morefresh absorbent to be supplied to the tank 128. On the other hand, ifthe measurement for the density of the separated dust material is toolow as compared to the set point, then the control system 146 may signaladjustment of the valve 152 to allow less fresh absorbent to be suppliedto the tank 128.

In accordance with a second embodiment of controlling the system 101,the control system 146 may control the level of supply of water viafluidly connected pipe 132 to system 101 by means of a control valve154. The amount of water supplied and the amount of fresh absorbentsupplied to system 101 influences the density of the separated dustmaterial, as measured by density meter 148, with greater amounts ofwater supplied increasing the density of the separated dust material,and greater amounts of fresh absorbent supplied reducing the density ofthe separated dust material.

According to similar principles as described hereinbefore with respectto the gas cleaning system 1, it is preferable to operate the gascleaning system 101 with as high separated dust material density aspossible, without causing operational disturbances. Hence, controlstrategies described hereinbefore with reference to FIGS. 1-4 may beutilized also in the gas cleaning system 101. Furthermore, asalternative to measuring using density meter 148 to obtain measurements,other direct and indirect separated dust material density measurementprinciples, as described hereinbefore with reference to FIG. 1, may beutilized for measuring the density of the separated dust material.According to one embodiment, the level of power supplied to the secondrotary discharger 124 for discharging separated dust material fromfabric filter 106 via fluidly connected pipe 127 could be measured, andthe measurement utilized by the control system 146 as an indirectmeasure of the density of the separated dust material. Furthermore, ameter for measuring an electrical property, such as conductivity,resistivity, or capacitance, of the separated dust material could beutilized to obtain measurements in combination with or as a replacementto the density meter 148. Still further, the level of power supplied tothe second rotary discharger 124 for discharging separated dust materialfrom fabric filter 106 via pipe 127 could be measured, and themeasurement utilized by the control system 146 as an indirect measure ofthe friction of the separated dust material.

It will be appreciated that numerous variants of the embodimentsdescribed above are possible within the scope of the appended claims.

Hereinbefore it has been described, with reference to FIG. 1, that thegas cleaning system 1 may comprise a mixer 8 in which a separated dustmaterial is mixed with water to form a moistened dust which is thenbrought into contact with the hot process gas 2 a in a contact reactor4. Furthermore, it has been described, with reference to FIG. 5, thatthe gas cleaning system 101 may comprise a mixer 108 in which absorbentmaterial is mixed with water to form an absorption liquid, which is thenbrought into contact with the hot process gas, such as flue gas 102, ina spray dryer chamber 105. It will be appreciated that the principles ofthe present method and gas cleaning system may also be applied to othertypes of gas cleaning systems, in which absorbent material is broughtinto contact, in a dry, moistened, or liquid form, with a hot processgas 2 a, with at least a portion of the absorbent material subsequentlyseparated in a solid form from the hot process gas 2 a thereby forming aseparated dust material. An example of one such other type of gascleaning system is described in U.S. Pat. No. 4,795,566 disclosing amethod in which fresh absorbent material and recirculated material aremixed, in a dry state, with hot process gases.

Hereinbefore it has been described that the electrical property metercould be a capacitance meter 76. It will be appreciated that it is alsopossible to utilize a conductivity meter or resistivity meter formeasuring an electrical property and obtaining a measurement relevant tothe conditions of the separated dust material.

Hereinbefore it has been described that the control system 46, 146initiates measuring an operating parameter of a device handling theseparated dust material, for example a level of power consumption of amixer 8 motor 44 or a level of power consumption of a discharger 24,124, and utilizes the measurement obtained for indirectly measuring thedensity and/or the friction of the separated dust material. The level ofpower consumed by a device handling the separated dust material is anindication of the resistance of the separated dust material handled.Such resistance may correlate to the separated dust material density,and may also correlate to the separated dust material friction.Depending on the composition of the separated dust material, thecorrelation between the measured level of power consumption and thedensity and/or friction of the separated dust material may be more orless strong. Hence, for some separated dust material compositions themeasured level of power consumption may have a strong correlation to theseparated dust material density, but a less strong correlation to theseparated dust material friction, and for other compositions ofseparated dust material, the situation may be the opposite. For sometypes of separated dust material, the measured level of powerconsumption may have a strong correlation to both the density and thefriction of the separated dust material. A skilled person can by routineexperimentation determine for a specific separated dust materialcomposition, if the level of power consumed by a device handling theseparated dust material correlates in a suitable useful manner to thedensity and/or the friction of the separated dust material.

To summarize, a gas cleaning system 1 for removing gaseous pollutantsfrom a hot process gas 2 a comprises a vessel 4 for bringing the hotprocess gas 2 a into contact with an absorbent material, and aseparating device 6 for separating at least a portion of the absorbentmaterial from the hot process gas 2 a to form a separated dust material.The gas cleaning system 1 further comprises a measuring device 48, 20,44, 76 for measuring, directly or indirectly, a dust parameter such as adensity, and/or a friction, and/or a hygroscopicity, and/or anelectrical property of the separated dust material, and a control system46 for controlling at least one operating parameter of the gas cleaningsystem 1 based on a measurement of the measured dust parameter.

While the invention has been described with reference to a number ofpreferred embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the presentinvention. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from the essential scope thereof. Therefore, it isintended that the invention not be limited to the particular embodimentsdisclosed as the best mode contemplated for carrying out this invention,but that the invention will include all embodiments falling within thescope of the appended claims. Moreover, the use of the terms first,second, etc. do not denote any order or importance, but rather the termsfirst, second, etc. are used to distinguish one element from another.

1. A gas cleaning system for removing gaseous pollutants from a hotprocess gas, the gas cleaning system comprising: a vessel for bringingthe hot process gas into contact with an absorbent material; aseparating device for separating at least a portion of the absorbentmaterial from the hot process gas to form a separated dust material; ameasuring device for measuring, directly or indirectly, at least onedust parameter selected from a group of dust parameters consisting of adensity of the separated dust material, a friction of the separated dustmaterial, a hygroscopicity of the separated dust material, and anelectrical property of the separated dust material, to obtain ameasurement; and a control system for controlling at least one operatingparameter of the gas cleaning system based on the measurement of themeasured dust parameter of the separated dust material.
 2. A gascleaning system according to claim 1, wherein the control system isadapted for comparing the measured dust parameter of the separated dustmaterial to a dust parameter set point, and controlling the at least oneoperating parameter to effect a change in the dust parameter of theseparated dust material based on such comparison.
 3. A gas cleaningsystem according to claim 1, further comprising: a density meter fordirectly measuring, as the dust parameter of the separated dustmaterial, the density of the separated dust material, to obtain ameasurement; and/or a scale for measuring a weight of a sample of adefined volume of the separated dust material to determine, as the dustparameter of the separated dust material, the density of the separateddust material, to obtain a measurement.
 4. A gas cleaning systemaccording to claim 1, further comprising: a motor driving a devicehandling the separated dust material, with the control system adapted todetermine the level of power drawn by the motor for indirectlymeasuring, as the dust parameter of the separated dust material, thedensity of the separated dust material, and/or the friction of theseparated dust material.
 5. A gas cleaning system according to claim 1,further comprising: an electrical property meter for measuring, as thedust parameter of the separated dust material, an electrical property ofthe separated dust material, to obtain a measurement.
 6. A gas cleaningsystem according to claim 1, further comprising: at least one of adevice selected from a group of devices consisting of a dust mixingdevice for mixing separated dust material with water to form a moisteneddust brought into contact with the hot process gas, and a spray dryerchamber with a liquid mixing device for mixing absorbent material withwater to form an absorbent liquid brought into contact with the hotprocess gas in the spray dryer chamber.